Nebular dead zone effects on the D/H ratio in chondrites and comets

Abstract

Context. Comets and chondrites show non-monotonic behavior of their deuterium-to-hydrogen (D/H) ratio as a function of their formation location from the Sun. This is difficult to explain with a classical protoplanetary disk model that has a decreasing temperature structure with radius from the Sun. Aims. We want to understand if a protoplanetary disc with a dead zone, i.e., a region of zero or low turbulence, can explain the measured D/H values in comets and chondrites. Methods. We use time snapshots of a vertically layered disk model with turbulent surface layers and a dead zone at the midplane. The disc has a non-monotonic temperature structure due to increased heating from self-gravity in the outer parts of the dead zone. We couple this to a D/H ratio evolution model in order to quantify the effect of such thermal profiles on D/H enrichment in the nebula. Results. We find that the local temperature peak in the disk can explain the diversity in the D/H ratios of different chondritic families. This disk temperature profile leads to a non-monotonic D/H enrichment evolution, allowing these families to acquire their different D/H values while forming in close proximity. The formation order we infer for these families is compatible with that inferred from their water abundances. However, we find that even for very young disks, the thermal profile reversal is too close to the Sun to be relevant for comets.

What is Neptune's D/H ratio really telling us about its water abundance?

“…We can first exclude models with D/H build lower than Earth's VSMOW value (vertical dashed line). This is the same value found in most CI chondrites (associated to the relatively volatiles rich C-type asteroids), and is simultaneously the lowest value measured in a comet (103P/Hartley measured with Herschel ) (Robert 2006;Ali-Dib et al 2015;Altwegg et al 2015). Hence, it is very unlikely that Neptune formed from building blocks with D/H build lower than this value, even if these blocks were rock-dominated.…”

“…Since methane has also been constrained in Jupiter and Saturn (Mousis et al 2014), its abundance in different planets can be compared and used as a robust tracer of atmospheric metallicity. The D/H ratio on the other hand has a long history of being used as a tracer E-mail: m.alidib@utoronto.ca for the formation temperature of ices (Mousis et al 2000;Ali-Dib et al 2015). This is because water vapors that went through high temperatures phases in the presence of the disk's H2 before re-condensing into ices should have lower D/H ratio than those which did not undergo this heating, due to the chemical reaction: HDO + H2 H2O + HD (1) that favor HDO's transformation into H2O at high temperatures.…”

D/H in the refractory organics of comet 67P/Churyumov-Gerasimenko measured by Rosetta/COSIMA

“…The discovery that Hartley 2 and 45P (both JFCs) had low D/H ratios (close to VSMOW) contradicts this simple picture. While models have been proposed to explain the unexpected range of water D/H values in comets (Yang et al 2013;Ali-Dib et al 2015), the high value seen by ROSINA in 67P (another JFC) is a challenge for models like these to explain. It has been suggested that the similar (Ceccarelli et al 2014) and in various objects in the Solar system, including the Earth (De Laeter et al 2003), primitive IOM from meteorites (Alexander et al 2012), bulk values for interplanetary dust particles (IDPs; Aléon et al 2001;Messenger et al 2003), ultracarbonaceous Antarctic micrometeorites (UCAMMs; Duprat et al 2010), Jupiter and Saturn (Ceccarelli et al 2014), Saturn's moon Enceladus (Ceccarelli et al 2014), Uranus and Neptune (Ceccarelli et al 2014), Oort-Cloud comets (Ceccarelli et al 2014;Altwegg et al 2017), and Jupiter-family comets (Ceccarelli et al 2014;Altwegg et al 2017;Drozdovskaya et al 2021).…”

Disentangling hot Jupiters formation location from their chemical composition

“…• The disk viscous evolution was not included in this work. It can affect the results indirectly via either its impact on the disk thermal structure (and thus the icelines locations) (Ali-Dib et al 2015), or through volatiles transport (by directly affecting the C/O ratio) (Ali-Dib et al 2014). Including these effects in future models is crucial for a more complete picture.…”